In recent times, access control systems for motor vehicles, such as the PASE system (Passive Start and Entry System) for example, have enjoyed widespread application. In this situation, systems of this type send electromagnetic waves on an inductive basis for data transmission between a motor vehicle and a data storage medium.
A known device for this purpose is shown in FIG. 1. In this case, a series resonance circuit 3 is supplied by way of a terminal or switch 2 which can be switched between two voltage levels with a square-wave voltage which corresponds to the resonant frequency of the resonance circuit 3. As a result of the resonance characteristics of the oscillating circuit, a resonance voltage can be achieved in this situation by way of a transmitting coil of the resonance circuit 3, which resonance voltage corresponds to a multiple of the voltage level of the DC voltage source 1.
In addition, as shown in FIG. 2, it is also known for a parallel resonance circuit 6 to be controlled by means of a square-wave alternating voltage using a capacitor C and a coil L via a resistor 5. In this situation, the square-wave alternating voltage is delivered to the resistor 5 by way of a terminal 2 which can be switched between two voltage potentials of a direct current source 4. In this situation, the switching frequency at the switchable terminal 2 corresponds to the natural frequency of the resonance oscillating circuit 6.
A further known device for generating electromagnetic waves is shown in FIG. 3. In this situation, an alternating voltage source 7 supplies a transformer 8 with a sine-wave voltage characteristic. The transformer 8 increases the input voltage and outputs it by way of a twisted cable 10 to a transmitting coil 9. As a result of the increased voltage at the terminals of the transmitting coil 9, their operating current is reduced. Losses on the feeder cable to the transmitting coil 9 are reduced as a result. Consequently, as a result of using the transformer 8, a spatial separation can be achieved between the alternating voltage source 7 as the power output stage and the transmitting coil 9.